BRL-CAD Primitives

This article provides an overview of various types of geometric primitive objects that can be added to a BRL-CAD geometry file. The shape, size, location and orientation of each such object are defined by a set of parameters/properties that are specific to its type, which are discussed in the corresponding section below.

For general discussions on using MGED to create primitive objects, view their properties, and modify or move them, see:

An arbitrary convex polyhedron (arb) is a geometric volume that is completely enclosed by a set of 3-dimensional planes. Each has a set of straight-edged, flat faces outlined by the intersections of those planes. The intersection of each pair of planes is a line whose intersections with other planes defines a pair of vertices. The line segment between those two vertices is an edge of the polyhedron that is shared by two faces. Each vertex is common to an equal number (at least three) of faces and edges.

For example, a rectangular parallelepiped is enclosed by three orthagonal pairs of parallel planes. Their intersections define six faces, each with four edges and four vertices. There are a total of 12 edges (each shared by two faces) and 8 vertices (each shared by three faces and three edges).

The BRL-CAD geometry file format defines two types of records for such polyhedra:

arbns are specified by a set of intersecting planes, each defined by four coefficients.

Although any polyhedron can be defined and stored as an arbn, the arb8 record type is more commonly employed because it is simpler to work with and still accommodates most constructive solid geometry applications.

An arb8 record is specified by a set of eight {X, Y, Z} vertices designated V1 through V8, which need not all be unique. BRL-CAD uses such records to represent polyhedra having four, five or six faces:

arb8 shapes have eight unique vertices. They represent hexahedra that have six quadrilateral faces sharing eight edges. In addition to simply specifying the {X, Y, Z} coordinates of those vertices, MGED provides easier ways to create the following specific types of hexahedra:

3ptarb shapes represent right quadrilateral prisms, which are extruded quadrilaterals having parallel ends connected by four rectangular sides.

box shapes represent parallelepipeds, whose faces comprise three pairs of equal parallelograms. Unlike a common box, those faces need not be rectangular—if they are, the enclosed volume is a rectangular parallelepiped.

rpp shapes represent rectangular parallelepipeds (also known as cuboids and rectangular prisms), whose faces comprise three pairs of equal rectangles. If one pair of faces are squares, the volume is a square prism. If all of them are squares, the volume is a cube (geometrically, there cannot be just two pairs of square faces).

arb7 shapes have seven unique vertices. They represent hexahedra that have four quadrilateral and two triangular faces sharing eleven edges. They can only be created by specifying the {X, Y, Z} coordinates of those vertices.

arb6 shapes have six unique vertices. They represent triangular prisms and truncated tetrahedra, which are pentahedra that have two triangular ends connected by three quadrilateral sides sharing nine edges. In addition to simply specifying the {X, Y, Z} coordinates of their vertices, MGED provides an easier way to create one specific type of hexahedron:

raw (right angle wedge) shapes are triangular prisms whose ends are parallel to each other. Interestingly enough, they don't seem to require any right angles. If the ends are perpendicular to the connecting edges, the shape is a right triangular prism and has rectangular sides. Presumably two of the rectangular sides of an actual right-angle wedge would also be perpendicular to each other.

arb5 shapes have five unique vertices. They represent quadrahedra, which are pentahedra that have a quadrilateral base and four triangular sides sharing eight edges. If such a volume has a rectangular base it is a rectangular pyramid, one with a square base is a square pyramid.

arb4 shapes have four unique vertices. They represent tetrahedra, which have four triangular faces sharing six edges. If all four triangles are equilateral, the shape is a regular tetrahedron.

The extruded bitmap (also referred to as EBM) is a solid based on a greyscale bitmap. The bitmap is an array of unsigned char values, see bw(5), and is extruded by some distance. The EBM solid requires the dimensions of the bitmap file (height and width in bytes), an extrusion length, and a transformation matrix to position the EBM. Each byte in the bitmap file is treated as the base of a cell that is extruded by the specified extrusion length. If the value of the byte is non­zero, then that cell is considered solid.

The particle solid is a lozenge-shaped object defined by a vertex, a height vector and radii at both ends. The body of the particle is either a cylinder or a truncated cone, depending on the values of the radii. Each end of the particle is a hemisphere of the specified radius.

Solids of type 'ars' (Arbitrary Faceted Solids) are defined using "waterlines". The following figure consists of a start point, some number of intermediate polygons, and an ending point. Each of the intermediate polygons have the same number of vertices and the vertices are numbered 1 thru N. In addition to the intermediate polygons a line will be created that begins at the start point, goes through each polygon at its vertex numbered 1, and terminates at the end point. This is repeated for each polygon vertex 2 thru N. The start point, polygons, and end point are each a "waterline".

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the ars shape takes the following values as input:

The number of points per waterline (the number of vertices on each intermediate polygon)

The vol solid is defined by a 3-dimensional array of unsigned char values. The solid requires a file of these values, the extent of the file (in bytes) in each dimension, the size of each cell, and high and low thresholds. Any value in the file that is between the thresholds (inclusive) represents a solid cell.